Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers onto a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) or extreme ultraviolet (EUV) spectral ranges. Conventional light wavelengths for DUV systems are currently 248 nm, 193 nm and sometimes 157 nm; EUV projection exposure apparatus currently use X-ray light with a wavelength of about 13.5 nm.
The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of structures, which is arranged on a mask, is thereby imaged onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than 1, such projection objectives are often also referred to as reducing objectives.
After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the mask. The photoresist still remaining is then removed from the other parts of the layer. This process is repeated until all the layers have been applied onto the wafer.
The performance of the projection exposure apparatus used is determined not only by the imaging properties of the projection objective, but also by an illumination system which illuminates the mask. To this end, the illumination system contains a light source, for example a laser operated in pulsed mode (DUV) or a plasma source (EUV), and a plurality of optical elements which generate light beams, converging on the mask at field points, from the light generated by the light source. The individual light beams desirably have particular properties, which in general are adapted to the projection objective and the mask to be imaged.
In order to be able to vary more flexibly the properties of the light beams striking the mask or the shape of the region illuminated on the mask, it has been proposed to use one or more mirror arrays, each having a plurality of adjustable mirrors, in the illumination system. The alignment of such mirrors is conventionally carried out by swivelling movements about one or two swivel axes. Such swivelling mirrors are therefore desirably fitted to suspensions which have one or two movement degrees of freedom. This may, for example, be achieved with solid-state articulations or with universal suspensions.
Mirror arrays, each having a plurality of adjustable mirrors, may also be used in projection objectives. For example, an array in a pupil plane of the projection objective may be envisaged in order to correct particular field-independent imaging errors.
The reflective layer systems, which are applied onto the supports of the adjustable mirrors, absorb an (albeit small) part of the incident light even in DUV projection exposure apparatus. In EUV projection exposure apparatus, the losses due to absorption are about 30%. The light absorbed by the mirrors heats them and, if sufficient dissipation of heat is not ensured, can lead to destruction of the reflective layer systems or other parts of the mirror units.